Products made from nonwoven webs such as bath tissues and facial tissues are designed to include several important properties. For example, the products should have a soft feel and, for most applications, should be moisture absorbent. The products should also have adequate stretch characteristics and should resist tearing. Further, the products should also have good strength characteristics, and should not deteriorate in the environment in which they are used.
In the past, many attempts have been made to enhance and increase certain physical properties of such products. Unfortunately, however, when steps are taken to increase one property of these products, other characteristics of the products may be adversely affected. For instance, the softness of sanitary paper products, such as tissue webs, can be increased by several different methods, such as by selecting a particular fiber type, or by reducing cellulosic fiber bonding within the product. Increasing softness according to one of the above methods, however, may adversely affect the strength of the product. Conversely, steps normally taken to increase the strength of a tissue web typically have an adverse impact upon the softness, the stiffness or the absorbency of the web.
In order to increase the softness of tissue webs without adversely affecting other characteristics of the web, those of ordinary skill in the art have devised different methods for drying the webs after the webs have been formed. For instance, in one embodiment, tissue webs have been dried using a heated dryer drum. In this embodiment, the web is pressed into engagement with the surface of a dryer drum to which it adheres due to its moisture content and its preference for the smooth surface of the drum. As the web is carried through a portion of the rotational path of the dryer surface, heat is imparted to the web causing most of the moisture contained within the web to be evaporated. The web can then be removed from the dryer drum by a creping blade which reduces internal bonding within the web and increases softness.
In an alternative embodiment, instead of wet pressing the tissue web onto a dryer drum and creping the web, the web is through air dried. A through air dryer accomplishes the removal of moisture from the tissue web by passing hot air through the web without applying any mechanical pressure.
A need remains, however, for an improved method for drying tissue webs. In particular, pressing a web on a dryer drum as described above has a tendency to compress the web causing the web to lose bulk. This problem is not experienced using a through air dryer. Through air dryers, however, have high energy and capital requirements and are relatively expensive to operate.
The present invention is directed to improvements in sanitary paper webs and to improvements in processes for making the webs in a manner that optimizes the physical properties of the webs. In particular, the present invention is directed to an improved process for drying tissue webs that is relatively economical and that produces tissue webs having high bulk and good softness.